This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to an electrode for a lithium electrochemical cell. The electrode comprises a first cathode active material of a relatively low energy density but of a relatively high rate capability and a second active material having a relatively high energy density but of a relatively low rate capability. The first and second active materials are short circuited to each other by contacting the opposite sides of at least one perforated current collector. Alternately, the electrode can comprise spaced apart first and second perforated current collectors, the second active material being at an intermediate position with the first active material contacting the opposite, and outer current collector sides.
A preferred form of the cell has the electrode as a cathode connected to a terminal lead insulated from the casing serving as the negative terminal for the anode. The present electrode design is useful for powering an implantable medical device requiring either a medium rate power source or a high rate discharge application. Suitable implantable medical devices include cardiac pacemakers, cardiac defibrillators, neurostimulators, drug pumps, hearing assist devices, and the like.
In that respect, it is a benefit of the present invention that the electrode has improved energy density in comparison to a prior art electrode. This is accomplished by pressing the active material using a cyclic application of pressure. Conventionally built electrodes are made using a static application of pressure. Not only does a cyclic pressing protocol increase the density of the active material, especially those of a carbonaceous nature such as CFx, but it also makes for a more planar electrode geometry. In conventional electrode designs, static pressing two disparate active materials onto opposite sides of a current collector often provides an electrode that is cupped. This is undesirable as it results in there being different impedance at the electrode periphery where spacing with the counter anode material is relatively close in comparison to the center of the electrode where inter-electrode spacing is greater. Cupping also adversely reduces the cell's effective volumetric energy density and frequently necessitates the addition of a remedial process step for flattening the electrode, which is not always successful.